The title originally referred to any elected king who had not yet been granted the Imperial Regalia and title of "Emperor" at the hands of the Pope. Later it came to be used solely for the heir apparent to the Imperial throne between his election (during the lifetime of a sitting Emperor) and his succession upon the death of the Emperor.

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The territory of East Francia was not referred to as the Kingdom of Germany or Regnum Teutonicum (Latinised from Old High Germandiutisc) by contemporary sources until the 11th century. During this time, the king's claim to coronation was increasingly contested by the papacy culminating in the fierce Investiture Controversy. After the Salian heir apparent Henry IV, a six-year-old minor, had been elected to rule the Empire in 1056 he adopted Romanorum Rex as a title to emphasize his sacred entitlement to be crowned Emperor by the Pope. Pope Gregory VII insisted on using the derogatory term Teutonicorum Rex ("King of the Germans") in order to imply that Henry's authority was merely local and did not extend over the whole Empire. Henry continued to regularly use the title Romanorum Rex until he finally was crowned Emperor by Antipope Clement III in 1084. Henry's successors imitated this practice, and were also called Romanorum Rex before and Romanorum Imperator after their Roman coronations.

Candidates for the kingship were at first the heads of the German stem duchies. As these units broke up over time, rulers of smaller principalities and even non-German rulers were considered for the position. The only requirements generally observed were that the candidate be an adult male, a Catholic Christian, and not in holy orders. The kings were elected by several Imperial Estates (secular princes as well as Prince-Bishops), often in the imperial city of Frankfurt after 1147, a custom recorded in the Schwabenspiegel code in about 1275.

After his election, the new king would proceed to be crowned as King of the Romans (Romanorum Rex), usually at Charlemagne's throne in Aachen Cathedral by the Archbishop of Cologne. Though the ceremony was no more than a symbolic validation of the preceding election result, it was solemnly celebrated. The details of Otto's coronation in 936 are described by the medieval chronicler Widukind of Corvey in his Res gestae saxonicae. The kings received the Imperial Crown from at least 1024, at the coronation of Conrad II. In 1198 the Hohenstaufen candidate Philip of Swabia was crowned Rex Romanorum at Mainz Cathedral (as was King Rupert centuries later), but he had another coronation in Aachen after he had prevailed against his Welf rival Otto IV.

At some time after the ceremony, the king would, if possible, cross the Alps, to receive coronation in Pavia or Milan with the Iron Crown of Lombardy as King of Italy. Finally, he would travel to Rome and be crowned Emperor by the Pope. Because it was rarely possible for the elected King to proceed immediately to Rome for his crowning, several years might elapse between election and coronation, and some Kings never completed the journey to Rome at all. As a suitable title for the King between his election and his coronation as Emperor, Romanorum Rex would stress the plenitude of his authority over the Empire and his warrant to be future Emperor (Imperator futurus) without infringing upon the Papal privilege.

Not all Kings of the Romans made this step, sometimes because of hostile relations to the current Pope and other times because the pressure of business at home, or warfare in Germany or Italy, made it impossible for the King to make the journey. In such cases, the king might retain the title "King of the Romans" for his entire reign.

The title Romanorum Rex became functionally obsolete after 1508, when the Pope permitted King Maximilian I to use the title of Electus Romanorum Imperator ("elected Emperor of the Romans") after he failed in a good-faith attempt to journey to Rome. At this time Maximilian also took the new title "King of the Germans" or "King in Germany" (Germaniae rex, König in Germanien), but the latter was never used as a primary title.

The rulers of the Empire thereafter called themselves "Emperors" without going to Rome or soliciting Papal approval, taking the title as soon as they were crowned in Germany or upon the death of a sitting Emperor if they were elected as heir to the throne.

The Holy Roman Empire was an elective monarchy. No person had a legal right to the succession simply because he was related to the current Emperor. However, the Emperor could, and often did, have a relative (usually a son) elected to succeed him after his death. This elected heir apparent bore the title "King of the Romans".[1]

The election was in the same form as that of the senior ruler, and theoretically meant that both men were equal co-rulers of the Empire. In practice, however, the actual administration of the Empire was always managed by the Emperor, with at most certain duties delegated to the heir.

^A junior King of the Romans was normally chosen only when the senior ruler bore the title of Emperor. Only on one occasion (1147-1150) was there both a ruling King of the Romans (King Conrad III) and a King of the Romans as heir (Henry Berengar). From the 16th century on, the senior ruler took the title of 'Emperor' from the time of his accession or succession; King of the Romans accordingly came to refer solely to the heir apparent.